This invention relates to systems for generating signals corresponding to the rotation of a shaft, disc, or other member, and more particularly to tachometer generators for attachment to primary power sources, which tachometer generators provide one or more proportional varying frequency signals without requiring external power.
Modern control and data processing systems include a wide range of velocity responsive signal generators used for timing, indication, or control functions. A particularly difficult problem is presented by tachometer generators that are required to be used with airborne vehicles, such as helicopters, because established standards permit only limited volumetric and weight values, while also requiring that a variable frequency A.C. signal be provided that is proportional to the rotation rate, and of sufficient power output to drive panel indicators without intervening amplifiers, so that maximum reliability can be achieved. The assignee of the present application has previously manufactured tachometer generators using a magnetic transducer sensing the passage of peripheral teeth on a magnetic wheel. While satisfactory for prior applications, more recent, stringent requirements have required that the tachometer generator be capable of confinement, within increasingly smaller volumes, and that up to four output signals be provided passively, each having significant individual power output levels. Such requirements could not be met by previous devices, in part because of mechanical considerations, but also because of space requirements. It is desirable, in these systems, to have a complete unit supported on a mounting pad on the prime mover housing, and while the unit must be rugged it must also fit within a cube of less than 3" on a side, thereby dictating that size and weight being minimized to the extent possible. It is also necessary for some applications to have more than one transducer generating an output signal and, for purposes of redundancy and safety, the transducer should be passive, requiring no external excitation, and function independently of any companion transducers. Also, the device must provide sufficient power to directly actuate panel indicators for the pilots of the vehicle.
In drum and disk memories, for example, it has long been known to use a track of magnetized spots or a recorded timing pattern, to provide clock signals for use in controlling the recording, reproduction and transfer of data. Variants of these systems for other applications have used toothed wheels whose inductive or magnetic coupling causes a change in signal which can be used to indicate the rotational velocity of a rotating member, and from this signal a digitally clocked or varying frequency A.C. signal can be generated. Such devices typically produce an output voltage of 7 or 8 volts at engine speeds of 4500 Rpm.
In addition to the sensing of peripheral teeth on a wheel, there are a number of speed indicators and tachometer generators that utilize disks or rotors with spaced apart apertures, and which sense passage of the apertures to generate a signal representative of rotational velocity. Illustrative of such patents is U.S. Pat. No. 2,630,529, which discloses an electrically conductive rotor with radially spaced apertures. At least two coils spaced on opposite sides of the disc are components of an oscillator circuit which can operate only when the aperture position permits coupling between the opposed coils. Thus, the output of the device is a series of constant frequency pulses which are proportional to disc rotation speed. This device does not lend itself to compact, independent operation since it requires sensing elements on each side of the rotating disc, and a power supply, along with a host of active circuit components, to operate the required oscillator. Any single power or component failure renders to device inoperative. Other representative patents include U.S. Pat. Nos. 3,938,112, 4,279,576, 4,370,614, 4,358,735, 4,447,517, and 4,775,777. These units, however, are also either excessively complex in requiring more than one sensor element to generate a signal, or require an external source of power, or generate some other signal than an A.C. signal whose frequency is proportional to rotational velocity.